Conventional reflector antenna devices having two reflectors include those disclosed in, for example, “A Simple Procedure for the Design of Classical Displaced-Axis Dual-Reflector Antennas Using a Set of Geometric Parameters”, IEEE Antennas and Propagation Magazine, Vol. 41, No. 6, pp. 64–72, in December, 1999, written by Tom Milligan. An example of the reflector antenna devices disclosed therein is shown in FIG. 12. As shown in FIG. 12, an electromagnetic wave radiated from a primary radiator 3 is reflected by an auxiliary reflector 1, reflected by a main reflector 2, and then radiated to a space. Also, because the configurations of the auxiliary reflector 1 and the main reflector 2 are determined so that the electromagnetic wave that has been radiated from a phase center 4 of the primary radiator 3 geometrical-optically passes through paths of 4-P-Q-R and 4-U-V-W, no electromagnetic wave geometrical-optically arrives in an area A where the auxiliary reflector 1 is projected on the main reflector 2 in parallel with a radiation direction of the electromagnetic wave by means of the main reflector 2.
Also, as another conventional reflector antenna, there has been proposed a reflector which is designed taking into consideration a wave influence on the basis of not geometrical-optical design but physical optics method as disclosed in, for example, Shinichi Nomoto and one other person, “Shaped Reflector Design for Small-Size Offset Dual Reflector Antennas”, Electronic information communication society article, November 1988, B Vol. J71-B, No. 11, pp. 1338–1344. In the reflector antenna, a radiation pattern is obtained on the basis of the physical optics method taking the wave influence into consideration, and the performances of both of a gain and a side lobe are optimized by using a non-linear optimization technique.
In the conventional reflector antenna device shown in FIG. 12, although no electromagnetic wave arrives in the area A geometrical-optically, the electromagnetic wave actually arrives due to the wave property of the electromagnetic wave. This phenomenon becomes remarkable as the size of the auxiliary reflector 1 becomes smaller in the wavelength ratio. The electromagnetic wave radiated from the primary radiator 3 is reflected by the auxiliary reflector 1, and undesirably contributes to a scattering wave due to the primary radiator 3, or a multiple reflected wave between the main reflector 2 and the auxiliary reflector 1, due to the influence of the electromagnetic wave that arrives in the area A. As a result, there arises such a problem that the characteristic deterioration of the antenna is induced.
Also, in the above-described document “Shaped Reflector Design for Small-Size Offset Dual Reflector Antennas”, although the antenna is designed according to the shaped reflector design based on the physical optics method, only the performance of the antenna is designed as an evaluation function. As a result, there arises such a problem that no attention has been paid to a risk of the deterioration of the performance due to an influence of the electromagnetic wave in the area in which the electromagnetic wave should not arrive geometrical-optically.